The present invention relates to a transfer device for a four-wheel drive motor vehicle, and more particularly to a device having a fluid operated multiple-disk friction clutch.
A four-wheel drive motor vehicle of the type in which a transfer device is provided for distributing the output torque of a transmission is disclosed in Japanese Patent Application Laid-Open 56-43035 filed by the assignee of this application. In the vehicle, the fluid operated clutch of the transfer device is provided between a drive element and a driven element of a power transmission system. The clutch comprises a drive drum and a driven drum, each having multiple-disks, and a piston oil chamber formed between drive drum and a piston. When oil is supplied to the piston chamber, the piston is urged by pressure of the oil to push disks of the drive drum against the disks of the driven drum. Thus, the opposite disks are engaged to transmit the torque of the drive element to the driven element. When the oil is drained from the chamber, the piston is returned to the initial position by a spring, so that the disks are disengaged with each other.
Since, the oil chamber is formed on a rotating shaft, when the shaft rotates at a high speed, the centrifugal force in the oil in the piston oil chamber is exerted on the piston, which causes various inconveniences.
More particularly, when the oil is drained from the piston chamber in order to disengage the clutch, the centrifugal force is still exerted on the piston, so that the clutch cannot be quickly disengaged. In addition, continuous partial engagement causes breakdown of the clutch by burning. Since the clutch is not perfectly disengaged, the four-wheel driving condition is kept. Accordingly, when the motor vehicle negotiates a corner, tight corner braking effect dependent on the difference between front-wheel speed and rear-wheel speed occurs to reduce the speed of the vehicle or to stop the vehicle.
In the vehicle having a control system for producing a desired torque distribution ratio signal for providing a desired torque distribution ratio for front and rear wheels, actual distribution ratio becomes different from the desired distribution ratio owing to the partial engagement of the clutch.
Japanese Patent Application Laid-Open 62-52249 discloses a clutch in which a centrifugal force oil chamber is provided opposite to the oil chamber interposing the piston. The lubricating oil is supplied to the centrifugal force oil chamber to generate the centrifugal force for canceling the centrifugal force of oil in the piston oil chamber. Accordingly, since the lubricating oil is continuously supplied from the oil pump to the centrifugal force oil chamber, the centrifugal forces in both chambers are balanced. However, the centrifugal forces vanish when the engine stops. Consequently, if the motor vehicle is towed by a wrecker to a garage, the centrifugal force of the oil occurs only in the oil chamber by the rotation of the clutch during the towing of the vehicle, because the oil in the centrifugal force oil chamber is drained and the oil is remained in the oil chamber. Thus, the clutch brakes down during the towing due to the engagement of the clutch. On the other hand, at the starting of the engine, it takes some time to charge the centrifugal force oil chamber with oil, so that the operation of clutch during the some time becomes unstable.